1. Field of the Invention
The present invention relates to a process for obtaining microspheroidal particles, more particularly silica-based microspheroidal particles. It also relates to microspheroidal silica particles of high specific surface. It relates further to catalysts containing chromium supported on these microspheroidal silica particles and to a process for polymerising alpha-olefins in the presence of these catalysts.
2. Description of the Related Art
Supported catalysts which can be employed for the polymerisation of alpha-olefins have been described in U.S. Pat. No. 2,825,721 (Phillips Petroleum) and are known under the name of catalysts of the Phillips type.
The catalysts described in this patent contain a chromium oxide dispersed on a support which may be silica and are activated by calcination in an oxidising atmosphere under conditions such that at least a proportion of the chromium is converted into hexavalent chromium.
Supported catalysts of this type have also been described which are capable of producing polymers exhibiting less high average molecular weight (that is to say characterised by higher melt indexes), which are more easily convertible by conventional processing methods. This modification is obtained by incorporating titanium oxide in the catalyst, besides chromium oxide. This incorporation may be carried out subsequently to that of chromium, by dispersing onto the support, via a non-aqueous medium, a titanium compound capable of being converted into oxide by calcination (U.S. Pat. No. 3,622,521 (Phillips Petroleum)). This incorporation can also be carried out by coprecipitating the titanium compound and the silica. The chromium compound can also be incorporated in this way (U.S. Pat. No. 3,887,494 (Phillips Petroleum)). The gels resulting from these coprecipitations are known, depending on their composition, under the name of silica-titanium "cogels", silica-chromium "cogels" and silica-titanium-chromium "tergels".
It is well known, furthermore, that the texture of these catalysts exerts a considerable influence on the morphology of the polymers obtained with their use. Because of this it has already been proposed to disperse chromium on so-called "microspheroidal" silicas. These are silicas whose particles, generally of spherical shape and of narrow particle size distribution, have a mean diameter or approximately between 0.5 and 200 microns.
Many methods for manufacturing these "microspheroidal" silicas have been described in the literature. Examples of these methods which may be mentioned are vaporisation, agglomeration and emulsion polymerisation.
In vaporisation techniques a solution of sodium silicate, optionally partially neutralised, is dried by evaporation at high temperature (Patents GB-A-1,077,908 and GA-A-1,094,798 (Grace & Co.)). The xerogel particles thus obtained are too fine in particle size for use as a catalyst support for alpha-olefin polymerisation.
The preparation of spheroidal silica particles which can be employed as catalyst supports for alpha-olefin polymerisation by spraying a hydrosol obtained by introducing a solution of an alkali metal silicate lengthwise and tangentially into a stream of aqueous inorganic acid has also been described (Patent Applications DE-A-2,540,279 and DE-A-2,540,278 (BASF)). The addition of the alkali metal silicate solution can also be performed at various points distributed along the stream of dilute inorganic acid (Patent Application DE-A-2,103,243 (BASF)). The hydrosol thus obtained is unstable and exhibits a lack of homogeneity due to changes in viscosity and in gelling time as a function of time and location. The production of silica microparticles by this process cannot therefore be uniform and/or reproducible with time.
Spheroidal silica particles can also be obtained by agglomeration of colloidal silica in a polymeric matrix of urea and formaldehyde (U.S. Pat. No. 4,010,242 (DuPont de Nemours)). Since the catalyst particles obtained by starting with these supports are brittle, they quickly break up during the polymerisation, to the detriment of the morphology of the polymers obtained with their use.
When emulsion polymerisation is used, a suspension of droplets of acidic sodium silicate solution is gelled in a viscous liquid in which neither the silicate solution nor the product of gelling are soluble (see, for example, Patent Application EP-A-170,334 (Shell Int. Res. Mij.)). This process makes it possible to obtain spheroidal particles of large size which must be treated mechanically to permit their use as catalyst supports for alpha-olefin polymerisation. This mechanical treatment takes place to the detriment of the morphology of the supports.
The present invention aims to provide a process for obtaining microspheroidal particles of good morphology, not exhibiting the disadvantages of the known processes.